1. Field of the Invention
The present invention relates to a vehicular multiple-speed mechanical transmission system, preferably a fully or partially automated mechanical transmission system, wherein the transmission has a plurality of upper ratios having a relatively small ratio step therebetween.
2. Description of the Prior Art
Multiple-speed vehicular mechanical transmissions for heavy-duty vehicles having 9, 10, 12, 13, 16, 18 or more forward gear ratios are well known in the prior art, as may be seen by reference to U.S. Pat. Nos. 4,754,665; 5,390,561 and 5,546,823, the disclosures of which are incorporated herein by reference. Such transmissions typically provided a relatively equal ratio step (i.e., the percentage change in ratio between adjacent ratios) across the entire ratio coverage (i.e., the difference between the highest and lowest speed forward ratios). The ratio steps were a compromise between providing a drivable vehicle and a desire to be in a ratio that will allow the engine to operate at or near a most fuel-efficient or otherwise desirable engine speed.
Automated mechanical transmission systems, including fully automatic systems, partially automatic systems and systems which automatically implement all or part of a manually requested shift, also are well known in the prior art, as may be seen by reference to U.S. Pat. Nos. 4,361,060; 4,648,290; 4,596,986; 4,873,881; 5,406,861 and 5,592,851, the disclosures of which are incorporated herein by reference. Automatic clutch controls for such systems also are well known in the prior art, examples of which are disclosed in U.S. Pat. Nos. 4,081,065; 5,403,249 and 5,624,350, the disclosures of which are incorporated herein by reference.
So-called continuously or infinitely variable transmissions ("CVT") for vehicular use also are known in the prior art. Examples of CVTs may be seen by reference to U.S. Pat. Nos. 5,108,352; 5,081,877 and 4,487,085. The advantages of a CVT is that the ratio may be continuously varied, not just in finite steps, to always cause the engine to operate at or very near the desired engine speed. These transmissions have not been commercially successful, especially for heavy-duty vehicles, as they tend to be limited in capacity, of unknown or unsatisfactory reliability, complicated and/or expensive.